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1-6
INTRODUCTION
Measuring Studio and Transmission Levels
It is far better to normalize levels in a playout system by making the average levels
of all elements identical, which means that they would all peak at the same level
when observed with a VU meter. the average output level of the playout system
should be set to about –18dBFS to keep OPTIMOD-PC AGC nominally in the center of
its range. This allows OPTIMOD-PC's AGC to work as unobtrusively as possible.
Moreover, if your system includes locally originated speech material, using a micro-
phone processor (like the dbx 286A, Symetrix 528E, or AirTools 6200/2x) will help
smooth the transition between live and recorded program segments.
Studio equipment (like mixers) and transmission equipment (like codecs) typically
use different methods of metering to display audio levels. The VU meter is an aver-
age-responding meter (measuring the approximate RMS level) with a 300ms rise
time and decay time; the VU indication usually under-indicates the true peak level
by 8 to 14 dB. The Peak Program Meter (PPM) indicates a level between RMS and the
actual peak. The PPM has an attack time of 10ms, slow enough to cause the meter
to ignore narrow peaks and under-indicate the true peak level by 5 dB or more. The
absolute peak-sensing meter (the type most common in codecs) shows the true peak
level. It has an instantaneous attack time, and a release time slow enough to allow
the engineer to read the peak level easily. All of OPTIMOD-PC's level meters are ab-
solute peak sensing.
Orban offers a free Loudness Meter application for Windows that incor-
porates a true peak-sensing meter, a VU meter, a PPM, and two types of
subjective
www.orban.com/meter.
Figure 1-3 shows the relative difference between the absolute peak level and the
indications of a VU meter and a PPM for a few seconds of music program.
The studio engineer is primarily concerned with calibrating the equipment to pro-
vide the required input level for proper operation of each device, and so that all de-
vices operate with the same input and output levels. This facilitates patching devices
in and out without recalibration and ensures that no part of the program chain will
Figure 1-3: Absolute Peak Level, VU and PPM Reading
loudness
meters.
It
ORBAN MODEL 1101
can
be
downloaded
ABSOLUTE PEAK
PPM
VU
from
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